f:Extension
extension: maximum cardinality of 'extension' is 0
value[x]: minimum cardinality of 'value[x]' is 1
Extension.value[x] 1
If a code for the unit is present, the system SHALL also be present (inherited)
There SHALL be a code if there is a value and it SHALL be an expression of time. If system is present, it SHALL be UCUM. (inherited)